The present invention relates to an apparatus and a method for controlling the supply of oil pressure to a friction engagement element such as a clutch in an automatic transmission.
Heretofore, in an automatic transmission for a vehicle, there is known a construction where, when engaging a friction engagement element such as a clutch, a pre-charge which is a process for quickly filling oil to the friction engagement element in advance, is performed, to thereby raise the oil pressure of the friction engagement element to that just before engagement (refer to Japanese Unexamined Patent Publication No. 5-106722).
However, there may be a case where, while performing the pre-charge based on a speed change request, a different speed change request is made giving rise to a requirement to conversely release the friction engagement element which was to be engaged by performing the pre-charge.
Heretofore, if as described above a different speed change request is made during pre-charging, an indicated pressure for the pre-charging is gradually reduced.
However during pre-charging, since the actual oil pressure is lower than the indicated pressure due to a oil pressure response delay, even though the indicated pressure is gradually reduced from the time when the different speed change request has occurred, the condition where the actual oil pressure is lower than the indicated pressure is maintained for a while.
In such a condition, the actual oil pressure continues to change upward, and as a result, the friction engagement element not to be engaged starts engagement, so that a speed change shock is likely to occur.
Furthermore, there is a situation where a speed change request occurs during pre-charging, requiring an even more rapid engagement of the friction engagement element.
In this case, the indicated pressure is further increased in a condition where the actual oil pressure is lower than the indicated pressure. Hence, the actual oil pressure is increasingly changed at a speed exceeding the rising speed of the indicated pressure, and due to this abrupt increasing change of the oil pressure, an engagement shock is likely to occur.
Accordingly, it is an object of the present invention to provide an apparatus and a method for controlling oil pressure, which can avoid the occurrence of a speed change shock when a different speed change request is made during pre-charging.
In order to achieve the above object, the present invention is constructed so that when a different speed change request is made during pre-charging, a processing based on the different speed change request is commenced after a processing has been performed to make an indicated pressure for a friction engagement element being pre-charged approach an actual pressure.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.